1. Field of the Invention
The field of the invention is collector-up, emitter-down HBTs capable of operating at microwave/millimeter frequencies.
2. Prior Art
The prior art is best represented by an article in IEEE Electron Device Letters, Vol. EDL-7, page 32, No. 1, Jan. 1986 by Sadao Adachi et al. entitled "Collector-UP HBTs Fabricated by Be.sup.+ and O.sup.+ Ion Implantations."
This article states that to successfully realize C-up HBTs, it is necessary to reduce parasitic current injection into the extrinsic base region. The authors apparently accomplish this by employing a high-resistivity layer buried in the extrinsic base-emitter junction region in order to block current injection from the emitter to the extrinsic base contact. This highly resistive layer buried in the extrinsic base-emitter junction region is formed by Be.sup.+ /O.sup.+ implantations. The Be.sup.+ implantation was carried out to make contact to the P+ gallium arsenide base layer in the intrinsic base area adjoining the collector. The 0.sup.+ implantation was also carried out with the same mask to enable the formation of a high resistivity layer in the N aluminum gallium arsenide emitter region underneath the P+ gallium arsenide base. In this way, parasitic current injection into the extrinsic base region is stated to be avoided.
However, this structure is not self-aligned, and there is no high frequency performance measured.